1. Technical Field
The present invention relates to a cylinder-to-cylinder variation abnormality detecting device that detects variation in air/fuel ratio for multi-cylinder engine.
2. Related Art
Conventionally, aftertreatment of engine exhaust has been performed using a catalytic converter, or simply “catalyst”, to reduce harmful substances included in the engine exhaust gas, such as hydrocarbon (HC), carbon monoxide (CO), and nitroxides (NOx). A three-way catalyst that oxidizes CO and HC and reduces NOx at the same time, so as to be converted into harmless carbon dioxide (CO2), water (H2O), and nitrogen (N2), has come to be used in common in recent years.
To obtain high conversion efficiency using a three-way catalyst, the air/fuel ratio (AFR) of the air-fuel mixture has to be controlled to a narrow range near a theoretical AFR (λ=1) by AFR feedback control. Accordingly, systems using such three-way catalysts may exhibit poor exhaust emissions if there is cylinder-to-cylinder AFR variation in the engine. Regulations in North America (On Board Diagnostics 2 (OBD-II)) stipulate onboard detection of cylinder-to-cylinder AFR variation abnormalities, since this is a factor causing poorer exhaust emissions.
One convention technique to detect such cylinder-to-cylinder AFR variation abnormalities is the rotational fluctuation method employing fluctuation in engine rotational angle speed (for example, see Japanese Unexamined Patent Application Publication JP-A No. 2012-154300) the AFR fluctuation method using fluctuation in the AFR of the air-fuel mixture detected by an AFR sensor disposed upstream of the catalytic converter (for example, see JP-A No. 2012-31774), and so forth.
Now, cylinder-to-cylinder variation abnormalities include a “rich malfunction” where the amount of fuel in the air-fuel mixture is too great (the air-fuel mixture is rich), and a “lean malfunction” where the amount of fuel in the air-fuel mixture is too small (the air-fuel mixture is lean). The sensitivity of the aforementioned rotational fluctuation method is low regarding rich malfunction. Accordingly, precision of diagnosis has been improved by combining this rotational fluctuation method with the AFR fluctuation method that has higher sensitivity regarding rich malfunction, to confirm an abnormality when a cylinder-to-cylinder variation abnormality has been detected by both methods.
However, if the responsivity of an AFR sensor becomes poor due to deterioration of the AFR sensor over time, or the like, the amplitude of the output waveform of the AFR sensor may becomes smaller, and cylinder-to-cylinder variation abnormalities become more difficult to detect. That is to say, an erroneous determination may be made by the AFR fluctuation method that there is no malfunction, even though cylinder-to-cylinder variation is occurring. In this case, the technique of combining the rotational fluctuation method with the AFR fluctuation method and confirming an abnormality if determination of cylinder-to-cylinder variation abnormality is made in both methods may result in erroneous determination that there is no malfunction, even though cylinder-to-cylinder variation is occurring.